The present invention relates to an apparatus for drying a coating layer which is formed on the surface of a web or an elongated strip.
Techniques for applying a liquid layer to a traveling web and drying the applied liquid layer and indispensable to the manufacture of a colored steel sheet, a PS (presensitized) plate (a plate or sheet for lithographicprinting in which the material of a base plate such as aluminum, paper, plastic or the like is coated with a photosensitive solution), a photographic film, or the like. They are extensively utilized in industry and are the subject of extensive research and development. In particular, the PS plate is manufactured in such a manner that the front surface of a web of, e.g., aluminum traveling continuously is subjected to mechanical, chemical and electrochemical treatments so as to be formed to have a roughened surface, an anodic-oxided surface and a hydrophilic surface, that the resulting surface is coated with the solution of a photosensitive resin in an organic solvent, and that after the coating is dried, the web is cut into a finished product. The drying step is important as a process for forming photosensitive layers.
In an apparatus for drying of this type, it is generally required not only to vaporize the organic solvent but also to control the remaining amount of high-boiling components. The reason is that the amount of the residual solvent affects the strength of the layers and the developing aptitude thereof at the time of use. That is, improper drying conditions are the cause of unclean printing ascribable to poor development and low printing durability ascribable to inferior layer strength. Such conditions severely detract from the function of the PS plate.
When broadly divided, the drying step consists of two subordinate steps. In the first subordinate step, principally the concentration of the solution is raised with the vaporization of the solvent so as to produce a soft layer. At this stage, drying is usually controlled under a mild drying condition in order to suppress the occurrence of orange peel and brushing which are known to be the drying defects of organic solvents. The second subordinate step is a stage for hardening the layer, for which the control of the amount of residual solvent is important. At this stage, the vaporization of the solvent has its rate determined by the diffusion migration rate within the layer. The drying equipment is also divided in this manner into a part for the first subordinate step and a part for the second subordinate step, the drying conditions whereof differ. In some cases, the drying step is made up of three subordinate steps by setting an intermediate condition between the first and second subordinate steps. In the prior art, all the subordinate steps rely upon hot air drying, and the conditions are optimized by controlling the temperature, the humidity and the flow rate of air. In general, the temperature is gradually raised from the beginning. Also, in terms of the equipment, the shape of a blowout port or the like is contrived for the purposes of coping with wind marks at the initial stage of drying, enlarging the coefficient of heat transfer, etc.
A drying apparatus as shown in FIG. 2 is employed in the prior art to implement the drying step.
FIG. 2 is a schematic view of the prior-art drying apparatus. An aluminum web 1 which travels continuously while being guided by guide rollers R is coated with a liquid containing a solvent by an applicator 2, and is introduced into a first subordinate drying zone 3. The first subordinate-step drying zone 3 is provided with a gas inlet 5 and a gas outlet 6. A gas supplied through the gas inlet 5 is distributed by a distributing vane 9 to form a uniform gas flow and is brought into contact with the surface of the coating formed on the aluminum web 1 by the coating step, so as to dry the coating, whereafter it is discharged through the gas outlet 6. The coating of the aluminum web 1 having reached the vicinity of the exit of the first subordinate-step drying zone 3 presents a soft film state. Subsequently, the elongated aluminum web 1 which travels continuously while being guided by guide rollers R is introduced into a second subordinate-step drying zone 4. This drying zone 4 is provided with gas inlet 7 and a gas outlet 8. A high-temperature gas supplied through the gas inlet 7 is forcibly expelled from a slit-type nozzle 10 to come into strong contact with the coating surface of the aluminum web 1. Thus, the solvent remaining in the coating vaporizes away, and the coating hardens. The gas is discharged through the gas outlet 8 after having contacted the coating.